This invention relates to the preparation of finely divided normally solid, synthetic organic polymeric thermoplastic resins.
Thermoplastic polymers in powder or finely divided form have a wide variety of commercial applications, such as for example, the dry powders have been used to coat articles in dry form by dip coating in either static or fluidized beds, by electrostatic coating, spraying or dusting and flame spraying. The powders are used in dispersed form in suitable liquid carriers to apply coatings by roller coating, spray coating, and dip coating to a variety of substrates such as, glass, ceramics, metal, wood, cloth, paper, paperboard, and the like. The finely divided polymers have also been successfully employed in conventional powder molding techniques. The fine powders have also been applied as paper pulp additives, mold release agents, wax polish, paint compositions, binders for non woven fabrics and finishes for woven fabrics.
There are basically four types of processes employed in the prior art for preparing finely divided polymer particles, i.e., mechanical grinding, solvent precipitation, dispersion and spray atomization of solutions or slurries.
Generally mechanical grinding employs conventional equipment to yield particles of irregular shape and wide size variation of from about 75 to 300 microns. The powders produced by this method are not suitable for applications where free flowing powders are required, since the irregular shapes may inhibit the flowability of these powders. The grinding of some polymers may be very costly because of the toughness of the resin, even when cryogenically cooled.
The precipitation technique generally entails dissolving the polymer in a solvent, followed by precipitation of the polymer in finely divided form through addition of a nonsolvent, evaporation of the solvent or a combination of the two. Problems in this process have included difficulty in manipulating the solvents, solvent removal, particle agglomeration which requires a great deal of energy and particles having irregular somewhat rounded shapes.
The dispersion method also is subject to high shear conditions. Frequently water in the dispersing medium and dispersing agents are used to facilitate the dispersion. Hence the powders produced by this technique generally incorporate some or all of the dispersing agent in the powder which can create undesirable changes in the original polymer properties, e.g., increased water sensitivity, loss of electrical insulating values, loss of adhesive capabilities, etc.
The final type of prior art process is the spray technique which is generally satisfactory for producing uniform non-agglomerated spherical particles, however, very speciallized equipment, usually nozzles operating under a limited range of conditions to prevent nozzle plugging are required. A substantial problem in spraying is the shearing of a polymer solution as it passes through the nozzle, premature precipitation of the polymer or too rapid volatilization.
Spray drying solutions of polymers is a difficult procedure. Large irregular particles or only fibers frequently result from sprayed polymer solutions. Not infrequently powder products from spraying are spheres with fine tails. Another frequent problem with spraying solutions has been a build up of precipitated polymer on the spray nozzles and generally sprayed solutions of polymer contain less than 30% by weight of the polymer. Furthermore, recovery and storage of the solvent can entail substantial equipment installations. Attempts to overcome these problems, generally have involved, random modification of process variables, changes of solvent and exotic spraying and atomizing equipment.
Some spray techniques, such as shown in U.S. Pat. No. 3,166,613 or Belgian Pat. No. 702,690, spray molten polymer, where molten polymer is pumped into a high velocity gas or vapor stream and discharged through a suitable nozzle. A principal problem encountered with this approach is the relatively high temperatures required to reduce the polymer viscosity to a satisfactory level for spraying, which can cause a substantial degradation in the molecular weight of the polymer.
The present invention has advantages over the prior spraying methods in that it may operate at lower temperatures than molten spraying techniques, hence less polymer degradation, yet will not involve the large recovery problems of solution spraying.